The present invention relates to a movable extrusion die useful for making honeycomb or cellular bodies in which the passages or channels are not always straight or parallel to the axis of extrusion.
Standard monolithic dies for extruding cellular structures are usually made using straight and parallel feed holes which communicate from the inlet side of the die to the slots in the outlet face of the die which is stationary, thus forming straight, parallel cells or channels. However, straight and parallel cells are not appropriate for certain applications. In some applications it is desirable to have the fluid make several passes through the channels before it is discharged. Multiple passes lead to more thorough heating and/or cleaning as the gas is allowed prolonged contact with the heat exchanger, catalyst or filter.
Extrusion dies can be formed in unitary die blocks by utilizing conventional machining and cutting techniques, EDM, or chemical machining. Traditional methods for making structures with non-parallel channels or passages generally require multiple steps. For example, in one approach a cellular ceramic body is cut and plugged so as to form non-parallel flow directions. In another approach often used for cross-directional flow structures such as heat exchangers and fuel cells, layers of green or fired sub-assemblies are formed by frit-bonding. This is the method often used for fuel cells where monolithic and planar structures contain non-parallel channels for fuel and air such as found in heat exchangers.
Heat exchangers are typically cross-flow structures formed by first extruding a honeycomb-like body of ceramic material from a die orifice. This extrusion results in a block of ceramic material having flow channels or cells which are generally of square or other rectangular cross-section, arranged parallel and adjacent to one another along the axis of extrusion. To form cross-flow heat exchangers, portions of the sides of the extruded ceramic block are commonly cut away to convert the ceramic block having straight-through passages into a ceramic block alternating between rows of straight-through flow, and Z-flow, L-flow, U-flow or other similar cross directional flow through the ceramic block. The cross-flow (Z-flow, L-flow, etc.) channels are typically made by sawing into the sides of some of the channels in the ceramic block and afterwards sealing the ends of these channels, thereby forming the cross-flow channels. In addition to heat exchangers, cross-flow structures are also useful for various other applications such as filtration, catalysis, oxygen production, and energy production. In cross-flow applications, gas or fluid flow in more than one direction through the structure.
Various methods have been disclosed for making cross-flow structures for example, by sawing and stacking. In the past, production of cellular structures having nonparallel channels or cells has required multiple steps. In one approach, a cellular ceramic body is cut and plugged so as to form non-parallel flow directions. In another approach, green or fired sub-assemblies of ceramic material are stacked and bonded together by sintering or frit-bonding. It has also been suggested to use the sawing technique to produce an L-flow cross-flow heat exchanger in which both flow directions through the heat exchanger follow an L-shaped path. When such sawing techniques are utilized to make cross-flow heat exchangers, very high precision extrusion geometries are required, as well as high precision cutting equipment, to arrive at a good quality finished cross-flow heat exchanger. Imprecision in either the extrusion or the cutting equipment can result in leakage paths between channels, which has a deleterious effect on heat exchanger performance. Further, because such heat exchangers are typically made by sawing into the side of the extruded ceramic body, it is very difficult to consistently achieve precise uniform cutting of the ends of the ceramic body. Such inconsistencies can result in leakage paths between adjacent channels. As mentioned above, such leakage paths can have a deleterious effect on heat exchanger performance.
Cross-flow heat exchangers having straight through flow channels in two directions have been disclosed in which layers having upstanding ribs thereon are laid one on top of another to form a heat exchanger having alternating layers of straight through flow channels, every other layer being arranged in a transverse direction to the one before it. The upstanding ribs of these layers in the green state are relatively weak, due in large part to their relative lack of support. Consequently, these methods sometimes result in the ribs being bent either prior to or during the stacking process. Furthermore, because each directional flow layer consists only of one layer of channels, the manufacturing process is relatively time consuming and labor intensive. Hitherto, traditional extrusion dies have proved inappropriate for producing cellular structures of the type described above where the channels are not necessarily parallel.
Co-pending, co-assigned U.S. Ser. No. 08/102,205 (Gardner et al.) has disclosed a method of preparing a cross-flow ceramic heat exchanger in which unidirectional layers of fluid flow passages are contacted with each other. Closed passages are disposed between two open and opposite ends, and the layers are arranged so that the passages of one layer are transverse to the passages of the other layer, thereby forming a laminated or layered structure. Batch materials used to extrude cross-flow blocks or portions are generally very stiff to prevent slumping or deformation of the extrudate as it exits the extrusion die. As a result, such batch materials require very high extrusion pressure, especially when forming very thin wall structures. To withstand such high pressures, it is necessary to use very strong and rigid extrusion dies. To overcome some of the above problems, recently in co-pending, co-assigned U.S. Ser. No. 08/132,923, (Faber et al.), a method has been suggested for forming self-supporting cellular structures by extruding relatively soft batches into a drying medium or by contacting the formed structure with a drying liquid immediately as the structure exits the extrusion die.
There continues to be a need for easier, more effective and less expensive methods for making cross-directional flow structures and other cellular structures in which the cells are not always parallel to the axis of extrusion. Accordingly, the object of the present invention is to provide an extrusion die and method of making geometrically complex cell directions such as cross-flow structures in which the cell directions are not always parallel to the axis of extrusion.